This allows us to flash hardware like the Retrode and other devices using chips
like the AT90USB1287.
The test files can be re-generated using the sample code found here:
https://github.com/hughsie/fwupd-test-firmware and are designed
to be used on the AT90USBKEY2 development board.
Notably, bootloaders for this class of device export an incorrect DFU interface.
Additionally, allow setting the buffer size for the UPLOAD to a larger size
than the defined device transfer size, which allows us to return the full
packet from the larger XMEGA devices.
